Electroceramic heating devices with metallized electrodes are generally made from doped barium titinate ceramics which have a sharp positive temperature coefficient of resistance. The ceramics are designed such that below a critical temperature, the resistance of the material remains at a low value and is essentially constant. When a particular temperature is reached, a crystalline phase change takes place in the ceramic and this change in crystalline structure is accompanied by a sharp increase in the resistance at the crystalline grain boundaries. The result of this crystalline change is an increase in the heater resistance of several orders of magnitude over a small temperature change. For example, barium titinate heaters with a room temperature resistance of 3.0 ohms can increase to 1,000 ohms or more during the crystalline phase change. The temperature at which the crystalline phase change takes place can be adjusted in the manufacturing process through the use of appropriate chemical additives and can attain temperatures as high as 300.degree. C., and even higher.
Commonly, current is carried to the device by means of wires that are attached to the metallized coatings that form the electrodes on the barium titinate body. When using devices that obtain temperatures as high as 300.degree. C., special solders or solder pastes are used, together with special fluxes. These special fluxes, may seriously harm the devices since they can impregnate the grain boundaries of the ceramic when the device is operated and cause detrimental changes to the resistivity of the heater. Moreover, when soldering techniques are used, quite frequently the device has to be preheated to receive the solder. This preheating takes a substantial amount of time in the manufacturing process and it is worthwhile to eliminate it.
While the soldering method is the most common way to attach leads to the heaters, other methods have also been used. Such methods include laser welding techniques which requires the use of extremely expensive laser equipment to produce a relative inexpensive product, and thus is not practical for the manufacture of many of the types of heaters envisioned by the present invention.